Amplifier coupling circuits



Dec; 7,1937. c. w. EARP AMPLIFIER COUPLING CIRCUITS Filed Feb. 28, 1935 M/l ENTOR C. W. EARP ATTORNEY Patented Dec. 7, 1937 UNITED sTATss PATENT OFFICE assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation oi New York Application February 28, 1935, Serial No. 8,602 In Great Britain July 23, 1934 2 Claim.

This invention relates to radio receiving apparatus and more particularly to means for controlling the selectivity of superheterodyne radio receivers.

With the increasing congestion of the various wave bands allocated to radio broadcast and the like, it is becoming more and more important that radio receivers should possess selectivity characteristics of a very high order. Extreme selectivity, however, inherently militates against good fidelity as the higher the selectivity the greater is the attenuation of the frequencies of the outer regions of the side-bands. In other words, a good fidelity characteristic cannot be simultane- 15 ously embodied in a receiver having a high selectivity characteristic.

In a superheterodyne receiver selectivity is largely obtained in the intermediate frequency circuits. .I'he transmission characteristics of 20 these circuits can be calculated and modified by the designer more or less at will and the band transmitted can, within reason, be any width the designer wishes.

If the coupling between the primary and sec- 5 ondary windings of the intermediate frequency transformers is increased, the transmission characteristic becomes wider and wider ultimately exhibiting the characteristic double hump of a band-pass circuit. It should be noted, however,

30 that the various selectivity characteristics obtained by modifying the coupling leaves the sides of the selectivity curves substantially parallel and they exhibit only a lateral displacement which, while broadening the nose of the curves,

35 does not seriously reduce the selectivity at say 20 kilocycles oil resonance.

In designing superheterodyne receivers it is usual to decide what selectivity characteristic is desirable and then to design intermediate fre- 40 quency units in which this characteristic is fixed and unchangeable during the operation of the receiver.

With the increasing demand for high fidelity when receiving, for example local stations, and

46 the insistence on extreme selectivity for obtaining distant stations free from interference, it is becoming desirable to provide means whereby the receiver can be modified at will to receive local stations with a broad selectivity character- 50 istic and distant stations with a. high selectivity characteristic.

It has already been proposed to employ a local distant switch which reduces selectivity by damping methods, e. g. shunt resistors, but this 66 method needlessly reduces the selectivity at say (Cl. TIL-119) 20 kilocycles oiI resonance because the skirts of the resonance curves do not remain parallel with increased damping, but the slope decreases thereby considerably reducing selectivity. It, however, the selectivity is varied by altering the coupling of the intermediate frequency coils, selectivity about 20 kilocycles ofl resonance, as has been pointed out, is not seriously impaired but the increase in band width transmitted permits a marked improvement in fidelity.

In accordance with a feature of the present invention, therefore, manual control means is provided for adjustably varying the physical relationship between the primary and secondary windings of an intermediate frequency transformer for the purpose of varying the coupling and thereby the band width transmitted.

According to another feature of the invention in a superheterodyne radio receiver provided with two or more intermediate frequency transformers, the primary and secondary windings of which are relatively displaceable for varying the coupling between them, a uni-control device is provided for manually adJusting the relative positions of the primary and secondary windings of each of the intermediate frequency transformers simultaneously.

One specific embodiment .of the invention will now be described with reference to the accompanying drawing in which:

Fig. l is a perspective view of two intermediate frequency transformers mounted upon the receiver chassis, a portion of which has been broken away to show the selctivity control mechanism and Fig. 2 is an elevation partly in section showing the control mechanism of Fig. l in greater detail.

The intermediate frequency transformers each 9. A helical spring I0 inserted between the hor izontally disposed side of the stirrup-shaped member 8 and the strip 3 tends to hold the primar coil 6 in its position of maximum spacing with respect to the fixed secondary coil l A horizontal spindle ll join'nalled in the side of the chassis 4 and in 9. depending bracket I2 which may be conveniently secured by two of the bolts 5, carries a number of eccentric cams is, one cam being provided for each intermediate transformer. The cams it are secured to the spindle M by grub screws M and are so positioned that their edges bear against the horizontal sides of the stirrup-shaped members 9.

The spindle Ill is journaled in a bushing l5 mounted in the side of the chassis 6, longitudinal movement of the spindle ll being prevented by two split pins l6, ll one of which engages the outer end of the bushing and the other of which bears on a washer 58 between which and the inner end of the bushing 85 is interposed a spring washer l9 which functions to hold frictionally the spindle ii in any position to which it is rotated. A pin 28 on the spindle H is adapted to engage a pair of stops 2i on the bracket 52 in order to limit the rotation of the spindle ii and cams iii to degrees. The end of the spindle ii projects from the side of the chassis 4 and is adapted to carry a control knob. The usual semi-fixed trimming condensers 22 are provided for tuning the pri mary and secondary circuits.

The operation of the device will be readily ap= parent. Rotation of the spindle ii and cams 53 in one direction raises the stirrup-shaped members 9 against the tension of the springs iii and in the other direction permits the springs i0 to depress the members t. The sleeves l are thus caused to slide upon the rods 2 and the primary coils 6 are moved in unison either nearer to or further from the secondary coils i. In this manner the selectivity of the receiver is varied and any desired fidelity characteristic can be obtained at the will of the operator.

It will be appreciated that the coupling of any number of intermediateirequency transformers may be mechanically controlled in this manner. Moreover, the invention is not limited to the preaionaav cise mechanical means described for effecting the variable separation of the coils and other arrangements will readily suggest themselves to those versed in the art.

What is claimed is:

1. In a superheterodyne radio receiver, a plurality of adjustable intermediate frequency transformers each comprising a fixed winding, a movable winding, a. rod axially supporting the fixed winding, a sleeve slidably mounted on the rod and axially supporting the movable winding, and a saddle-shaped member attached to said sleeve. a narrow mounting strip extending through the saddle-shaped members and having the rods mounted thereon and a driving mechanism operating on all of said saddle-shaped members in unison to simultaneously vary the spacing and consequently the electro magnetical coupling of the fixed and movable windings.

2. in a superheterodyne radio receiver, a plurality of adjustable intermediate frequency transformers each comprising a fixed winding, a movable winding electromagnetically coupledthereto, a rod axially supporting the fixed winding, a

sleeve surrounding the rod and having the movable winding axially supported thereon, a plate member attached to said sleeve, and a saddleshaped member having its ends attached to the plate member, a. mounting strip extending through the saddle-shaped members and having the rods secured thereto, a helical spring mounted between each of the saddle-shaped members and the mounting strip, a control shaft, and a plurality of cam members mounted on said shaft, the surface of one of said cam members bearing on each of said saddle-shaped members whereby the operation of said control shaft simultaneously varies the spacing of the movable and fixed windings of all of the transformers.

v 1"" 11- WELIAM I 

